Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-08-15
2001-09-11
Olszewski, Robert P. (Department: 2167)
Motor vehicles
Steering gear
With electric power assist
C701S041000
Reexamination Certificate
active
06286621
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric power steering apparatus for lightening the steering force of a driver by applying the drive force of an electric motor directly to a steering system.
2. Description of the Related Art
One known electric power steering apparatus employs a differential transformer as a steering torque sensor. In this electric power steering apparatus, a clockwise or counter-clockwise steering torque generated by turning a steering wheel is detected as an electric differential output generated in the differential transformer. The magnitude of the steering torque is detected in the form of an absolute value of the differential output while the direction of the steering torque is detected in polarity (±) of the differential output.
Another known electric power steering apparatus employs an operational amplifier as a steering torque detector, which amplifier is driven by a single power supply of 5 V. The operational amplifier is set to have a virtual ground point of 2.5 V so that a differential output of 0 in the differential transformer (steering torque being 0) is detected as a detection output of 2.5 V, the differential output of negative polarity (steering torque being in counter-clockwise direction) is detected as 0 to 2.5 V, and the differential output of positive polarity (steering torque being in clockwise direction) is detected as 2.5 V to 5.0 V.
In the steering torque detector, the state in which no steering torque is applied to the steering wheel (steering torque being 0) is set as a neutral point and the output of 2.5 V of the operational amplifier, corresponding to the neutral point, is set as a center value.
Still another known electric power steering apparatus is provided with a direction inhibitor for inhibiting, on the basis of a steering torque signal, the feed of a motor control signal to a motor drive to thereby determine whether or not a direction signal of the motor control signal, outputted from a control system for controlling the drive of the electric motor, and a direction signal outputted from the direction inhibitor are consistent. In the case of inconsistency, the assist by the electric motor, as opposed to the steering direction, is inhibited.
The direction inhibitor is set with two action reference values corresponding to the detected outputs of the steering torque detector in the clockwise and counter-clockwise directions. When the detected output in the clockwise direction exceeds the action reference value in the counter-clockwise direction, the electric motor assist in the counter-clockwise direction is inhibited. When the detected output in the counter-clockwise direction exceeds the action reference value in the clockwise direction, the electric motor assist in the clockwise direction is inhibited. With this construction, the electric motor assist is inhibited in response to a motor control signal outputted from a control system, when a CPU (or microprocessor) constructing the control system is troubled, for example, to invite the inconsistency between the direction signal, as detected by the direction inhibitor, and the direction signal of the motor control signal, as outputted from the control system.
As a result, an unnatural behavior of the vehicle is prevented by inhibiting such an electric motor assist against the will of the driver that the electric motor will assist in the counter-clockwise direction although the driver is turning the steering wheel in a clockwise direction.
In the conventional electric power steering apparatus, however, the assist time period by the electric motor for a torque value less than a predetermined value (e.g., 30 kgf·cm) is shortened where the steering force is set heavily in a high vehicle speed range. As a result, when achieving a steering damper effect by making an assist in the direction opposed to the torque direction, there arises a problem such that a reduced level of damper effect is produced, because the motor output is inhibited by a direction discriminator of the control system when the steering torque is at a predetermined or higher value and when the motor output direction and the torque value direction are different.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an electric power steering apparatus which comprises an electric motor for applying an assist torque to a steering system, a steering torque detecting sensor for detecting the magnitude and direction of a manual steering torque acted upon the steering system, a control unit for outputting a motor control signal for controlling the electric motor on the basis of a signal fed from the steering torque detecting sensor, the control unit having an assist inhibitor for inhibiting the drive of the electric motor when the output of the steering torque detecting sensor and the direction of the assist torque of the electric motor are different for a predetermined or longer time period, and a motor drive for driving the electric motor on the basis of the motor control signal of the control unit.
One specific example of the assist Inhibitor includes a torque direction determining section for determining the consistency/inconsistency between the direction of the manual steering torque detected by the steering torque detecting sensor and the direction of the assist torque by the electric motor, a timer for timing the duration of the inconsistency determination output from the torque direction determining section, and a time comparator for comparing the inconsistency duration fed from the timer with a predetermined time period.
Another specific example of the assist inhibitor includes a torque direction determining section for deter mining the consistency/inconsistency between the direction of the manual steering torque detected by the steering torque detecting sensor and the direction of the assist torque by the electric motor, a timer for timing the duration of the in consistency determination output from the torque direction determining section, a predetermined time setting section for setting a predetermined time period in accordance with the output from the steering torque detecting sensor, and a time comparator for comparing the inconsistency duration fed from the timer with the predetermined time period.
By virtue of the assist inhibitor, It becomes possible to inhibit the feed of the assist torque by the electric motor to the steering system when the duration of inconsistency between the direction of the manual steering torque and the direction of the assist torque is equal to or longer than a predetermined time period, thereby achieving the damper effect at least for the predetermined time period. After lapse of this time period, the feed of the assist torque by the electric motor to the steering system can be Inhibited reliably.
Further, in the electric power steering apparatus, when the duration of inconsistency between the manual torque direction and the assist torque direction is shorter than the predetermined time period, the assist torque of the electric motor to the steering system is not inhibited, thereby achieving continued control and damper effect upon repeated steering operations.
Moreover, in the electric power steering apparatus, the predetermined time setting section for setting the predetermined time period in accordance with the output from the steering torque detecting sensor is provided to make it possible to set the time period till the interruption of the output of the electric motor in accordance with the output from the steering torque detecting sensor. As a result, it becomes possible to set the time period in conformity with the characteristics of the electric motor, the vehicle weight and the transient characteristics of the vehicle.
According to a second aspect of the invention, there is provided an electric power steering apparatus which comprises an electric motor for applying an assist torque to a steering system, a steering torque detecting sensor for detecting the magnitud
Mukai Yoshinobu
Noro Yoshiki
Fischer Andrew J.
Honda Giken Kogyo Kabushiki Kaisha
Merchang & Gould P.C.
Olszewski Robert P.
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